The proteins of the normal human lens and cataracts of various etiologies are being characterized. These studies include identifying the major protein species and the quantitating changes in the levels of these proteins in cataracts. An enzyme that protects proteins against thiol-dependent oxidative inactivation has been identified in bovine, rat, and primate lens. The enzyme has been purified and identified y sequence analysis. Seventy percent of the amino acid sequence has been obtained. The interaction of copper, iron, and zinc with a number of proteins, including lens crystallins, has been studied. All three metals alter the solubility of the lens crystallins and many of the other proteins studied. Copper and iron are metals generally thought to be involved in the metal-catalyzed oxidation of proteins. In addition, these studies show them capable of inducing aggregate formation.